Left panels: W(Ca IIλ3934) in the upper panel and W(Na Iλ5891) in the lower panel as a function of the colour excess E(B − V). The green dots are the data from Wild & Hewett (2005). In all panels the green dashed lines correspond to the observations of Murga et al. (2015) in the interstellar and circumgalactic media of the Milky Way. The red dashed lines are extrapolations of these relations. The blue line in the W(Na I) versus E(B − V) plot is an empirical relation given by Poznanski et al. (2012): log(E(B − V)) = 2.16 × W − 1.91. Right panels: same as left panels with log N(H I) on the x-axis.